Deep-bed fluid bed combustors which provide good desulfurization of coal have been known; however, these devices have the drawbacks both of requiring great freeboard (above the bed) height and of blowing out unburned bits. Such combustors are discussed in H. Nack et al., "Fluidized-Bed Combustion Review," Vol. II, at pp. 346-47 of Fluidization Technology (Proceedings of the International Fluidization Conference at Asitomar, California, June 15-20, 1975), Hemisphere Publishing Co., Washington, D.C., 1976.
Also known have been shallow-bed fluidized bed coal combustors in which pieces of coal burn while floating (in effect) on a shallow bed of (e.g.) sand; these devices give good combustion and low freeboard height, but provide poor desulfurization. They are described in "Fluidized Combustion," Proceedings, Vol. 1, p. B3-2, Symposium Series No. 1 (1975), published by The Institute of Fuel, London.
Shallow-bed combustors use much coarser coal than do deep-bed combustors.